As a method for controlling a silicon single crystal pulling speed, there has been conventionally known a first method which directly feeds back a deviation of a diameter during a silicon single crystal pulling operation to a single crystal rod pulling speed or a second method which directly feeds back a deviation of the diameter to a heater temperature.
On the other hand, with high integration of a semiconductor device in recent years, a design rule is further finely distinguished, and a subtle defect on a silicon wafer as a material greatly affects a device yield. Therefore, when a wafer is manufactured by slicing a pulled single crystal rod on a surface orthogonal to an axis thereof, a wafer which does not have a defect on an entire surface thereof must be produced. Thus, assuming that a temperature gradient in an axial direction in the vicinity of a solid-fluid interface during a single crystal rod pulling operation is G (° C./mm) and a pulling speed is V (mm/minute), it is important to set the pulling speed over a full length so that V/G becomes constant and perform a control to realize this set pulling speed. In order to maintain V/G over the full length, since the temperature gradient G is large at a top portion on the initial stage of the single crystal pulling operation and the temperature gradient becomes small from the top portion to a predetermined pulling position, when a pulling speed appropriate for a change in the temperature gradient G is set, the pulling speed at the top portion must be increased, and it is general to set the pulling speed to be gradually decreased to the predetermined pulling position.
However, since this difference in the set pulling speed becomes a difference in an actual pulling speed, there is a problem that improving the controllability in the top portion by the first method increases a fluctuation in diameter of any portion other than the top portion, and improving the controllability in any portion other than the top portion increases a fluctuation in diameter of the top portion.
Further, in the second method, since a control breadth of a heater temperature and a change breadth and a change time of a melting temperature vary depending on a positional relationship between a liquid level and the heater or a melt quantity, a control over the heater temperature becomes very difficult. A change direction of the actual pulling speed does not match with a correction direction of the heater temperature depending on situations, and a fluctuation in diameter may be possibly increased.
In order to solve such problems, there is disclosed a method for manufacturing a silicon single crystal which calculates a control value of a pulling speed of a silicon single crystal rod, calculates a correction quantity of a heater temperature by comparing the control value of the pulling speed with a set pulling speed in order to obtain a set output of the heater temperature and controls a diameter of the silicon single crystal rod before subjecting the control value of the pulling speed to a pulling speed span restriction and subjecting the calculated control value of the pulling speed to a span restriction (Japanese Patent Application Laid-open No. 2001-316199).
In the conventional method for manufacturing a silicon single crystal disclosed in Japanese Patent Application Laid-open No. 2001-316199, however, since the pulling speed control value is compared with the set pulling speed and a deviation obtained from this comparison is fed back to a heater temperature before span-restricting the pulling speed control value, there is a possibility that a correction quantity of the heater temperature cannot follow up a deviation of an actual pulling speed and a fluctuation in diameter of the single crystal rod is increased.
It is an object of the present invention to provide a method for manufacturing a silicon single crystal, by which a set pulling speed for a silicon single crystal rod is set so that V/G becomes fixed, an actual pulling speed can be accurately controlled so that the actual pulling speed matches with the set pulling speed and a fluctuation in diameter of the single crystal rod can be thereby suppressed.
It is another object of the present invention is to provide a method for manufacturing a silicon single crystal, by which generation of a defective portion in a silicon single crystal rod can be reduced or avoided by predicting presence/absence of generation of the defective portion when pulling up the silicon single crystal rod and correcting a set pulling speed and a set heater temperature.